We are working toward a new perspective in understanding and manipulating the pathophysiology of beta thalassemia, a common and debilitating inherited anemia. A hallmark of this disorder is excessive free ? hemoglobin (Hb), an unstable protein that generates reactive oxygen species (ROS) and forms cytotoxic precipitates. We identified alpha hemoglobin stabilizing protein (AHSP), an abundant erythroid protein that enhances the solubility of free alphaHb and limits its biochemical reactivity. Ahsp-/- mice exhibit hemolytic anemia with Hb precipitates and excessive ROS. Moreover, loss of AHSP exacerbates beta thalassemia in mice, raising the possibility that altered AHSP function or expression could modulate beta thalassemia phenotypes in humans. Preliminary data support both mechanisms. First, we discovered a naturally occurring missense mutation, AHSP N75I, which impairs protein function and is associated with unexpectedly severe beta thalassemia in two pedigrees. Second, AHSP appears to be a quantitative trait locus (QTL) whose expression varies considerably between different individuals. Moreover, reduced AHSP expression associates with more severe clinical disease in several independent studies of small beta thalassemia cohorts and pedigrees. Together, these findings lead to the hypothesis that AHSP is a genetic modifier of beta thalassemia. We will test this by analyzing thalassemic populations for AHSP gene mutations, including N75I, and determining their effects on gene expression and/or protein function. In addition, we will study how variations in erythroid AHSP expression affect nascent alpha-Hb pools, oxidative stress and clinical severity in beta thalassemic patients. Our findings should provide new insights into the mechanisms of normal erythropoiesis and the pathophysiology of beta thalassemia. Ultimately, this information could provide a basis for developing novel therapeutic approaches to mitigate the toxicities of free alpha-Hb in beta thalassemia.